1. Field of the Invention
The present invention relates to a television signal receiving tuner.
2. Description of the Related Art
A conventional television signal receiving tuner will be described with reference to a block diagram of FIG. 3 and a selectivity characteristic diagram of FIG. 4.
In FIG. 3, a television signal receiving tuner comprises a mixer 21, a local oscillator 21, an SAW filter 23, an intermediate frequency circuit 24, and the like. In recent years, the television signal receiving tuner is also formed on an integrated circuit for miniaturization, and the mixer 21, local oscillator 22, intermediate frequency circuit 24, and the like are integrally formed on an integrated circuit 25.
The integrated circuit 25 has a plurality of terminals 26 to 33 so as to be connected to other circuits.
A television signal RF received by an antenna (not shown) passes through a resonance circuit (not shown) and is supplied to the mixer 21 in the integrated circuit 25 via the input terminal 30. The television signal RF is frequency converted by the mixer 21 and the local oscillator 22 and is outputted as an intermediate frequency signal IF from the first terminal 27 to the outside of the integrated circuit. The intermediate frequency signal IF outputted from the first terminal 27 passes through the SAW filter 23 and is supplied again to the integrated circuit 25 via the second terminal 28. The intermediate frequency signal IF which is supplied again to the integrated circuit 25 is properly amplified by an intermediate frequency amplifying circuit 34 in the intermediate frequency circuit 24 to a level necessary for a detecting operation of a video detecting circuit (not shown) at the post stage, and the resultant signal is outputted from the integrated circuit 25 via the output terminal 29.
The selectivity characteristic of the SAW filter 23 has, as shown by A in FIG. 4, a sharp attenuation characteristic out of an intermediate frequency band BW in order to eliminate interference from adjacent channels. An attenuation amount of each of a video intermediate frequency signal +P which is in an adjacent band on the upper side and a sound intermediate frequency signal xe2x88x92S which is in an adjacent band on the lower side from a video intermediate frequency signal P is 45 dB.
The insertion loss of the SAW filter 23 is 15 dB.
On the other hand, since the distance between adjacent terminals in the integrated circuit 25 is short, the t terminals are not isolated from each other sufficiently.
For example, the isolation between the first terminal 27 for outputting the intermediate frequency signal IF from the integrated circuit 25 to the outside and the second terminal 28 for again supplying the signal into the integrated circuit 25 is 40 dB.
In the configuration, when it is assumed that the level of the video intermediate frequency signal P in the first terminal 27 is a reference value 0 dB, since the insertion loss of the SAW filter 23 is 15 dB, as shown in FIG. 4, the level of the video intermediate frequency signal P in the second input terminal 28 is xe2x88x9215 dB.
On the other hand, since the isolation between the first terminal 27 and the second terminal 28 in the integrated circuit 25 is 40 dB, the video intermediate frequency signal +P which is in the adjacent band on the upper side and the sound intermediate frequency signal xe2x88x92S which is in the adjacent band on the lower side in the first terminal 27 appear in the second terminal 28 at the level attenuated by 40 dB from the reference value 0 dB (refer to the straight line B in FIG. 4).
Even when a signal of a adjacent channel is attenuated by 45 dB by the SAW filter 23, the difference between the level of the video intermediate frequency signal P at the second terminal 28 and the level of each of the video intermediate frequency signal +P in the adjacent band on the upper side the sound intermediate frequency signal xe2x88x92S in the adjacent band on the lower side is decreased to 25 dB.
Consequently, interference of the signals of the adjacent channels easily occurs and the video quality deteriorates.
It is therefore an object of the invention to realize a television signal receiving tuner capable of improving the video quality of a television receiver by substantially enlarging the difference between the level of a video intermediate frequency signal P in an intermediate frequency band and the level of each of a video intermediate frequency signal +P which is in the adjacent band on the upper side and a sound intermediate frequency signal xe2x88x92S which is the adjacent band on the lower side.
According to the invention, in order to achieve the object, there is provided a television signal receiving tuner comprising: an integrating circuit including a mixer and an intermediate frequency circuit which frequency convert at least a received television signal to an intermediate frequency signal; an intermediate frequency amplifying circuit which is separate from the integrated circuit and has an amplification degree; and an SAW filter which is separate from the integrated circuit and attenuates adjacent bands out of the intermediate frequency band of the television signal by a predetermined attenuation amount, wherein the integrated circuit has at least a first terminal and a second terminal and outputs the intermediate frequency signal obtained by the frequency conversion by the mixer from the first terminal, and the intermediate frequency signal is serially passed through the intermediate frequency amplifying circuit and the SAW filter and supplied from the second terminal to the intermediate frequency circuit.
According to the television signal receiving tuner of the invention, the amplification degree of the intermediate frequency amplifying circuit is set to be equal to or higher than a value which is obtained by adding insertion loss of the SAW filter to the absolute value of a difference between the isolation between the first terminal and the second terminal and the attenuation amount of the SAW filter.
According to the television signal receiving tuner of the invention, the intermediate frequency amplifying circuit is provided at the front stage of the SAW filter.